In shell and tube heat exchangers, such as nuclear steam generators, tube supports are used to minimize tube vibration induced by the fluid flowing on the shell side of the exchanger. These tube supports may be drilled plates, machined plates with various clearances around the tube, or lattice supports built from metal strips or bars. In any event, there exists areas of close clearance between the tube and support. One of the most widespread and serious problems observed in nuclear steam generators is "denting", a term used to describe the tube distortion which results when a tube support corrodes to the point where it squeezes the steam generating tubing. As this phenomenon begins, the tube tends to "lock in" at the support plate. The tubes may become locked in before any measurable denting has been detected. If a tube locks in at a support plate, it may result in stresses within the tube which could cause considerable damage. It is therefore desirable to have a means for determining if selected nuclear steam generator tubes are in a locked in condition.